1. Field of the Invention
The present invention relates to a sheet aligning device, a sheet processing device, and an image forming apparatus.
2. Description of the Related Art
Sheet processing devices, such as finishers in image forming apparatuses, are required to have sheet aligning mechanisms for precisely aligning sheet recording media (hereinafter, referred to as “sheet(s)” as appropriate) discharged on sheet trays. For example, sheet processing devices that include sheet aligning mechanisms near sheet discharge portions are known. Those sheet aligning mechanisms align the sheets on the sheet trays.
A conventional sheet processing device is disclosed in Japanese Patent Application Laid-open No. 2002-167104. This technology provides a sheet processing device that aligns a sheet discharged from a discharging unit in a sheet discharging direction. A stacking unit of the sheet processing device is provided with an end fence, and a sheet is properly aligned when a leading edge of the sheet, which is discharged on a sheet tray by the discharging unit, hits the end fence. The sheet processing device includes a returning unit, that is, a rotation member that aligns a sheet by applying stress to the sheet on the sheet tray so that the sheet is moved toward the end fence and hits the end face. The returning unit can be set on arbitrary positions along the sheet discharging direction.
Furthermore, Japanese Patent Application Laid-open No. 2002-179326 discloses another conventional sheet processing device. This technology provides a sheet processing device that performs sorting and aligning of sheets on a sheet tray using less driving power regardless of how many sheets are present on the sheet tray. The sheet processing device includes a discharging unit that discharges a conveyed sheet, a stacking unit on which the sheet discharged by the discharging unit is stacked, and an aligning unit that aligns the sheet stacked on the stacking unit. The aligning unit includes a mechanism for aligning a sheet at a predetermined position in a direction (a shift direction) perpendicular to a sheet discharging direction and another mechanism for aligning the sheet at different positions in the shift direction perpendicular to a sheet discharging direction with respect to each stack of sheets.
Moreover, Japanese Patent Application Laid-open No. 2002-356270 discloses still another conventional sheet aligning device, an image forming apparatus, and a sheet processing device. This technology provides an aligning mechanism that can align a sheet on a sheet tray even when the sheet is curled. The sheet aligning device include a discharging unit that discharges a conveyed sheet, a stacking unit on which the sheet discharged by the discharging unit is stacked, and an aligning unit that aligns the sheet stacked on the stacking unit. The aligning unit aligns a sheet by sandwiching side edges of the sheet in a direction parallel to a sheet discharging direction. The aligning unit is rotatably supported on a rotation axis such that a base point of the aligning unit can rotate within a predetermined range. A position of the aligning unit is adjusted so that the aligning unit can sandwich the side edges of the sheet by controlling amount of rotation of the rotation axis.
However, in the conventional sheet processing devices, sheets are aligned when sheets are discharged on the sheet tray. Therefore, if curled sheets or Z-folded sheets are stacked on the sheet tray, a trailing edge of the stack of the sheets becomes thick, and a positional relationship between a shift jogger and side edges of the stack of the sheets is disturbed. This leads to degradation of precision of sheet alignment.
Furthermore, in the sheet processing device disclosed in Japanese Patent Application Laid-open No. 2002-356270, the shift jogger includes a detecting unit that detects a state of a sheet stacked on the stacking unit. However, this method is effective only in a shift mode and it is ineffective in a normal stacking mode.